Recently, interests in energy storage technologies are increasing. As batteries are used in electric vehicles as well as in mobile phones, camcorders and notebook computers, research and development of the batteries grows briskly. An electro-chemical battery is the field of attention in this aspect, and in particular, with a recent trend of miniaturization and light weight of electronic equipments, the electrochemical battery meets the demand for a rechargeable battery of small size, light weight and high capacity.
The electrochemical battery or an electric double-layer capacitor primarily uses a liquid electrolyte, in particular, an ion-conducting organic liquid electrolyte, in which salt is dissolved in a non-aqueous organic solvent.
However, use of the liquid electrolyte increases the possibility that an electrode material may be deteriorated and an organic solvent may be volatilized, and threatens safety of a battery. For example, a battery may be burned out due to an increase in ambient temperature or temperature of the battery itself. In particular, in the case of a lithium secondary battery, gas is generated in the battery due to decomposition of a carbonate organic solvent and/or a reaction between an organic solvent and an electrode during charge/discharge, resulting in the expanded thickness of the battery. If the battery is stored at high temperature, this phenomenon is accelerated to further increase an amount of gas generation.
The continuously generated gas increases the internal pressure of the battery, and finally the battery of an angled shape is blown up in a specific direction and then a specific surface of the battery is deformed in the center. And, the gas makes a local difference of adhesion on an electrode surface in the battery, so that an electrode reaction does not occur uniformly over the entire electrode surface. As a result, the battery cannot avoid a reduction in performance and safety.
Generally, it is known that safety of a battery increases in the order of a liquid electrolyte, a gel polymer electrolyte and a solid polymer electrolyte, and performance of the battery decreases in such an order. Because the solid polymer electrolyte is poor in battery performance, batteries including the solid polymer electrolyte are not commercially produced.
Meanwhile, because the gel polymer electrolyte is excellent in electrochemical safety as mentioned above, the gel polymer electrolyte can uniformly maintain the thickness of a battery and ensure excellent adhesion with an electrode due to an intrinsic adhesive strength of a gel. A conventional method for manufacturing a battery using the gel polymer electrolyte includes the following two methods.
As one method, a polymerizable monomer and a polymerization initiator are added to a liquid electrolyte, in which salt is dissolved in a non-aqueous organic solvent, to prepare a composition. The composition is introduced into a battery case, in which a cathode, an anode and a separator are assembled in the type of roll or stack, and is gelled (crosslinked) under proper conditions of temperature and time, to manufacture a battery including a gel polymer electrolyte.
However, the above-mentioned method needs a separate process for maintaining the temperature required for gelation, resulting in loss of time and economical efficiency. And, according to composition of a polymerizable monomer or a polymerization initiator, gelation may occur at room temperature without heating, but it is a problem that the gelation may occur before introducing the composition including the polymerizable monomer, the polymerization initiator and the liquid electrolyte into the battery case.
As another conventional method, a polymerizable monomer and a polymerization initiator are added to a liquid electrolyte, in which salt is dissolved in a non-aqueous organic solvent, to prepare a composition. The composition is coated on a separator, and gelled using heat or UV. The separator is assembled with a cathode and an anode in a battery case to manufacture a battery. A liquid electrolyte is introduced into the battery.
However, this method needs a process of applying heat or UV for gelation, and the gel-coated separator absorbs water, resulting in deterioration in performance and stability of the battery.
Further, a polyethylene membrane that serves as a separator in the conventional art has a high thermal shrinkable rate, and thus, when temperature increases under conditions of abnormal use, a short circuit occurs between the cathode and the anode, resulting in reduction in stability of the battery.